1. Field of the Invention
The present invention relates to silicate-dextrin adhesive compositions and process for producing the same.
2. Reported Developments
Aqueous alkali metal silicates, particularly sodium silicates, are very old in the art and have many technical applications including their use as binders in coatings and as adhesives for solid fiber and corrugated boxes and cartons and plywood manufacture. They are versatile adhesives, combining convenience for use with strength and rigidity. However, in their use in the manufacture of paper products, such as paper tubes, they do not provide sufficient initial tack and may cause breakdown of the manufacturing process.
Dextrin and modified dextrins have also been used as adhesives for various applications, including many uses in the paper industry. While these adhesives quickly develop initial tack, they are difficult to clean from the processing machinery. They also do not have the strength and rigidity which the silicate-type adhesives have and their use is rather limited in the paper industry where strength and rigidity is required.
In order to provide the desirable characteristics of quick initial tack, rigidity and strength in adhesives intended for use in the paper industry, aqueous alkali silicates were combined with dextrin, as in U.S. Pat. No. 3,433,691. In so doing, the prior art has found that at least about one percent of dextrin is required to improve the initial tack quality over that obtained with straight silicate adhesives. While the initial tack requirement could be improved with the addition of more than one percent dextrin, thereby enabling a fast, automatic process in the manufacture of paper laminates, another problem arose. When the composition contained more than about 0.5 percent dextrin, the dextrin "floated out", rising to the surface of the adhesive composition. This floating out was observed both at the time of application of the adhesive to paper products and on storage of the adhesive. The floating out has resulted in faulty paper products and clogging of the complex dispensing apparatus.
U.S. Pat. No. 3,767,440 report that the floating out problem has been overcome by the addition to the aqueous alkali metal silicate/dextrin adhesive composition of a clay slurry consisting of a swelling type clay and a kaolin type clay. The addition of the clay slurry renders the adhesive composition thixotropic and prevents floating out of the dextrin from the adhesive composition.
The prior art also addresses the need of flame-resistant adhesive compositions, such as disclosed in U.S. Pat. No. 4,066,463, which discloses the combination of asbestos fibers with alkali metal silicate, starch, carboxymethylcellulose and dextrin.
Other references are directed to the use of vegetable proteins in silicate containing adhesives, such as: U.S. Pat. Nos. 2,457,108 and 2,681,290. These adhesives require heat on application to the surfaces to be joined and to form the necessary bond between the surfaces. Still other references disclose the use of cellulosic material, such as wood pulp to control viscosity, for example, U.S. Pat. No. 3,241,990, or soy polysaccharides, for example, U.S. Pat. No. 4,437,893 in combination with alkali metal silicates.
The object of the present invention is to provide a silicate-dextrin based adhesive composition having superior initial tack, adhesive strength and other desirable characteristics heretofore not quite satisfactorily provided for use in the paper industry including but not limited to tube winding, corrugated and comer boards.